Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOx storage catalyst storing NOx contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich, arranging a fuel addition valve in the engine exhaust passage upstream of the NOx storage catalyst, and using a combination of several temperature raising means such as the supply of additional fuel into the combustion chamber or the addition of fuel from the fuel addition valve so that the NOx storage catalyst is raised in temperature most suitably when raising the temperature of the NOx storage catalyst (see Japanese Patent Publication (A) No. 2003-120392).
In this regard, when making the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst rich so as to make the NOx storage catalyst release the NOx, supplying additional fuel into the combustion chamber so as to make the air-fuel ratio of the exhaust gas rich, compared with adding fuel from the fuel addition valve to make the air-fuel ratio of the exhaust gas rich, results in the oxygen concentration of the exhaust gas as a whole falling, so NOx being released from the NOx purification catalyst and reduced better. Therefore, to make the NOx purification catalyst release the NOx, it is desirable to make the air-fuel ratio of the exhaust gas smaller by additional fuel supplied into the combustion chamber.
On the other hand, when arranging an SOx trap catalyst able to trap the SOx contained in the exhaust gas upstream of the NOx storage catalyst, this SOx trap catalyst prevents the flow of SOx into the NOx storage catalyst. When using such an SOx trap catalyst, it is necessary to avoid the SOx trap catalyst releasing SOx as much as possible, but depending on the air-fuel ratio of the exhaust gas flowing into the SOx trap catalyst, there is a danger that SOx will be released. In this case, in the same way as an NOx storage catalyst, in an SOx trap catalyst as well, the supply of additional fuel into the combustion chamber enables SOx to be released easier than the addition of fuel from the fuel addition valve.
Therefore, to simultaneously achieve both a good action of release of NOx from the NO storage catalyst and prevention of release of SOx from the SOx trap catalyst, it is desirable to supply additional fuel into the combustion chamber and add the insufficient amount of fuel from the fuel addition valve so that the air-fuel ratio of the exhaust gas exhausted from the combustion chamber becomes the smallest in the range where the SOx trap catalyst does not release SOx.